The chemotactic signal transduction pathway of Escherichia coli is representative of a large family of signal transduction pathways that are distributed throughout the Bacteria and Archaea. These pathways are already known to mediate chemotaxis and phototaxis in free-swimming and surface-associated bacteria, but are being recognized increasingly to regulate multicellular aggregation, pilus expression, and biofilm formation - medically relevant phenomena that are characteristic of pathogenic microbes. The relative simplicity of the E. coli system, combined with the well-developed and sophisticated tools for its study, provides a strong rationale to determine in detail the sensory physiology, biochemistry and structural biology of the E. coli pathway. Transmembrane signaling in the E. coli system occurs in the context of a heterogeneous cluster, or array, of receptor proteins that are associated with the cytoplasmic adaptor protein, CheW, and the central signaling kinase, CheA. Attractant binding to the receptors causes subtle conformational changes in the receptor that are somehow propagated across the membrane to produce two effects (i) inhibition of CheA, and (ii) stimulation of receptor methylation. Kinase inhibition exhibits a positive cooperativity indicative of regulation by a cluster of receptors. The key to understanding the detailed mechanism lies in understanding the manner in which these clusters of receptor complexes are assembled and remodeled during function. In this project we will investigate the interactions among receptors and with the signaling proteins in samples of successively greater complexity (in vitro to in vivo), to determine what structures and interactions are critical to the control of kinase and methylation activity. The simplest system, active complexes of the cytoplasmic domain, CheA, and CheW assembled on vesicle surfaces, will be studied in the greatest detail, with a combination of biochemical and biophysical tools (activity assays, disulfide crosslinking, fluorescence, and solid-state NMR). Studies of assemblies of complexes of the intact receptor in vesicles will determine how the interactions in the array are modulated by the other receptor domains and by ligand. Finally, in vivo fluorescence studies will investigate remodeling of receptor complexes and arrays during function. The approaches developed and insights gained will be applicable to other systems in which changes in both conformation and subunit association play a role in the mechanism of transmembrane signaling.